Question

They are very similar, but there are minor differences.

When talking about people in general, we would think an average normal human being. When talking about traits or personality, a general person would have ambitions of getting a job, having fun, getting married, and living life in general.

On the other hand, the word "ordinary" would look at more objective traits such as how one physically looks. But depending on the context, it can also be used as the definition above.

So in retrospect, you use "general person" when talking about people in general (their traits, or whatever the context of the conversation is) while "ordinary person" would be used to describe someone, usually physical traits.